Cutting insert and milling tool

ABSTRACT

A milling tool includes a cutting tool insert, roughly plate-shaped and having a rake face on an upper face and a flank on a side face and a main cutting edge at an intersection ridge line part at the intersection of the rake face and the flank, fitted in the periphery of a substantially cylindrical holder removably and so that an axial rake angle of the cutting edge ridge line of the main cutting edge is positive. When the cutting tool insert is rotated about the axis of rotation of the holder, the shape of the locus of rotation of the main cutting edge in a section parallel to the axis of rotation is made an approximate circular arc shape swelling outward with progress from the ends to the center.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a milling tool used as a rotating bitin a machine tool such as a machining center.

2. Description of the Related Art

There has been known end mills in which a cutting insert having a shapeof an approximate parallelogram is fitted into a tip part of a holderused as a rotating tool in machining with machine tools such asmachining centers. The cutting insert is fitted into the holder so thata long side is disposed at the periphery of the end mill as a maincutting edge, a short side is disposed at the tip of the end mill as aflat cutting edge, and the main cutting edge slants at an angle, calledthe axial rake angle, to the axis of rotation of the holder.

Conventionally, a cutting insert in a plan view of which a main cuttingedge is formed in a straight line have been used. However, when thecutting insert is fitted in a holder with an axial rake angle asdescribed above and shoulder work is carried out, the machined wall faceis not flat and becomes a convexly curved face swelling outward withprogress from the sides toward the center so that the part machined withthe central vicinity of the main cutting edge projects slightly outward.In machining where flatness is important, the machined wall facebecoming a convexly curved face projecting with progress from the sidesto the center like this not only impairs the quality of the appearanceof the product, but also, because a re-finishing step becomes necessary,entails excessive machining time and machining cost, and has beenproblematic from the point of view of machining efficiency.

In this connection, to resolve this problem and obtain a smooth machinedsurface, in Japanese Examined Patent Publication JP-B2 3085686 it isdisclosed that in a plan view of a cutting insert itself a main cuttingedge swells outward, thereby it is made possible to form into a straightline a sectional form of a locus of rotation of the main cutting edge ofthe end mill which is cut by an imaginary plane which includes an axisof rotation and is parallel therewith and therefore a machined wall faceafter machining can be formed to be not so uneven.

However, when a milling tool fitted with a cutting insert whose maincutting edge swells as disclosed in Japanese Examined Patent PublicationNo. 3085686 is used, although when the dimensions of the cutting insertare accurately controlled and the cutting insert is mounted to theholder properly it is possible to control the locus of rotation of themain cutting edge to an accurate cylindrical shape right to the cuttingedge ends, in practice, due to variation of the fitting accuracy betweenthe holder and the cutting insert and variation of dimensional accuracy,a difference often arises between the turning radii of the ends of themain cutting edge of the cutting insert fitted in the holder, and therehas been the problem that when multi-cut machining in shoulder work orgrooving work is carried out in this state, large steps are formed atthe joins between passes in the machining of multiple passes in themachined surface. And, in practice bending of the holder occurs duringmachining due to cutting load, and in this case, even if the cuttinginsert was mounted in a position such that the locus of rotation assumesthe ideal cylindrical surface shape, steps will be still formed in themachined wall face. That is, obtaining a machined face with absolutelyno steps has been, in practice, almost impossible.

SUMMARY OF THE INVENTION

The invention has been made to solve these kinds of problems of relatedart, and an object of the invention is to provide a milling tool withwhich, in cutting work it is possible to make small steps, i.e.,unevenness, in a machined wall face of a workpiece that arise due tomulti-cut machining in shoulder-cutting and grooving work, and machine aproduct with a machined wall face with a higher machined face roughnessall the way from one end to the other end of the cutting edge.

To solve the problems mentioned above, the invention provides a cuttinginsert to be fitted to a substantially cylindrical milling tool holder,comprising a substantially plate-shaped insert body, a rake face formedon an upper face of the insert body, a flank formed on a side face ofthe insert body, and a main cutting edge at an intersection ridge linepart between the intersection of the rake face and the flank, whereinwhen the holder is rotated around a center of axis while the cuttinginsert is fitted in the circumferential periphery of the milling toolholder, of a locus of rotation of the main cutting edge, a middle partin the holder axial direction projects further outward in the holderradial direction than end parts in the holder axial direction.

According to the invention, a milling tool having a main cutting edge atits periphery is formed by the cutting insert being fitted to themilling tool holder. The milling tool discretely intermittently cuts aworkpiece to make a grooving or shoulder cutting by making contact withthe workpiece while rotating about its axis. In addition, it is possibleto increase a depth from a surface to a machined bottom face of theworkpiece by carrying out so-called multi-cut machining, in which aworkpiece is cut at multiple different levels in the holder axialdirection.

In the invention, a middle part of the main cutting edge in the holderaxial direction projects further outward, in the holder radial directionthan end parts of the main cutting edge in the holder axial direction.In other words, the holder axial direction end parts of the main cuttingedge are more inward than the holder axial direction middle part withrespect to the locus of rotation of the main cutting edge.

It is difficult to eliminate a difference in the holder radial directionbetween distances from the axis of the holder to the holder axialdirection end parts of the main cutting edge caused by a dimensionalerror of the cutting insert, mounting error of the cutting insert to theholder, or bending of the holder. When there is a difference in theholder radial direction between distances from the axis of the holder tothe holder axial direction end parts of the main cutting edge, comparedto the case where the difference in the holder radial between distancesfrom the axis of the holder to the holder axial direction end parts ofthe main cutting edge is zero, amount of overcutting is small at one ofthe holder axial direction end parts of the main cutting edge an un-cutpart arises, and large at the other. Herein an amount of overcuttingmeans a depth of insertion of the cutting edge into a workpiece byovercutting in a direction perpendicular to the axis of rotation of theholder. When the amount of overcutting is small, cutting amount of aworkpiece by the main cutting edge is small in the directionperpendicular to the axis of rotation of the holder. When the amount ofovercutting is large, cutting amount of a workpiece by the main cuttingedge is large in the direction perpendicular to the axis of rotation ofthe holder.

According to the invention, by multi-cut machining being carried out,the holder axial direction middle part of the main cutting edge can cutaway any part to be cut of a workpiece which has undergone insufficientcutting caused by either of the holder axial direction end parts of themain cutting edge. Accordingly, it is possible to reduce the steps inthe machined wall face formed due to a difference in the holder radialdirection between distances from the axis of the holder to the holderaxial direction end parts of the main cutting edge and thereby make themachined wall face even. And it is possible to make small the influenceof the above-mentioned difference in the holder radial direction betweenthe holder axial direction end parts of the main cutting age, on themachined surface roughness of the machined wall face after cutting.Accordingly it is possible to reduce the steps in the machined wall facewhich may arise at the joins between cutting passes, and the machinedsurface roughness of the machined wall face after cutting can beimproved. And the cutting insert can be used in the machining ofmaterials where a machined wall face with a high machined surfaceroughness is required.

In the invention it is preferable that the sectional shape of the locusof rotation of the main cutting edge cut by an imaginary planecontaining the axis of rotation is an approximate circular arc shapeformed by the holder axial direction middle part swelling furtheroutward in the holder radial direction than the holder axial directionend parts.

According to the invention, most of the un-cut part of the workpieceformed by the holder axial direction tip end part of the main cuttingedge in a previous cutting operation of a relatively small cuttingamount can be cut by the holder axial direction middle part of the maincutting edge, and the machined surface roughness of the machined wallface after cutting can be improved further.

In the invention it is preferable that the amount of holder radialdirection swell w of the locus of rotation of the main cutting edge fromthe holder axial direction end parts to that portion of the holder axialdirection middle part which projects farthest out in the holder radialdirection is 0.02 mm or more and 0.06 mm or less.

When the amount of swell w in the holder radial direction is less than0.02 mm it is not possible to absorb variation of the main cutting edgeposition, that is, differences in the holder radial direction betweendistances from the axis of the holder to the holder axial direction endparts of the main cutting edge, arising when the cutting insert isfitted to the holder, and the steps in the machined wall face becomelarge. And when the holder radial direction swell w exceeds 0.06 mm, themachined wall face becomes too concave and the steps in the machinedwall face become large as a result of that.

With respect to this, in the invention, as a result of the radialdirection swell amount w being 0.02 mm or More and 0.06 mm or less, thedifference in the holder axial direction between distances from the axisof the holder to the holder axial direction end parts of the maincutting edge can be absorbed without the radial direction swell wbecoming too large. Accordingly it is possible to make small the stepsin the machined wall face and stably obtain a high-quality machined wallface.

Furthermore, in the invention it is preferable that, of the locus ofrotation of the main cutting edge, the holder axial direction centerpart projects farthest outward in the holder radial direction.

In this case, whether the holder axial direction tip end part of themain cutting edge projects further outward in the holder radialdirection than the holder axial direction base end part or the holderaxial direction base end part projects further outward in the holderradial direction than the holder axial direction tip end part, in eithercase the machined surface roughness of the machined wall face can beprevented from becoming high. This is particularly useful in cases whereit is not possible to ascertain which of the holder tip end part and theholder base end part of the main cutting edge is positioned furtheroutward in the holder radial direction.

Furthermore, in the invention it is preferable that of the holder axialdirection middle part of the locus of rotation of the main cutting edge,a holder tip end side portion may project farthest outward in the holderradial direction.

During cutting, when the holder tip end part bends with respect to theholder base end part away from the workpiece, at the location in contactwith the workpiece, the holder axial direction tip end part of the maincutting edge withdraws from the workpiece in the holder radial directionto inward of the holder axial direction base end part of the maincutting edge.

According to the invention, compared to the rest of the main cuttingedge, a holder tip end side portion of the holder axial direction middlepart of the main cutting edge projects farthest outward in the holderradial direction. Accordingly, even when the holder bends duringcutting, the projection amount by which the holder axial direction baseend part of the main cutting edge projects further outward in the holderradial direction than the holder axial direction tip end part of themain cutting edge can be made small. Therefore, the affect of bending ofthe holder can be reduced, and the machined surface roughness ofmachined wall faces of the workpiece can be improved.

Furthermore, in the invention, it is preferable that the cutting inserthas a bottom face that abuts with a seat face of the milling tool holderwhen the cutting insert is fitted to the milling tool holder, and themain cutting edge slopes toward this bottom face with progress from theholder tip end part toward the holder base end part.

According to the invention, the main cutting edge is formed in a twistedshape having a twist angle. Accordingly, even when a degree of slope atwhich the seat face of the holder slopes with respect to the axis of theholder is small, a large axial rake can be provided. In this way it ispossible to lower the cutting resistance encountered by the main cuttingedge during cutting.

Furthermore, in the invention, it is preferable that the cutting inserthas a corner R cutting edge continuous with the main cutting edge, andthe radius of curvature r of this corner R cutting edge is 0.4 mm ormore and 4.0 mm or less.

According to the invention, as well as chipping of the cutting insertbeing prevented, the steps in the machined wall face are made small anda high-quality machined surface is stably obtained.

Furthermore, in the invention, it is preferable that the cutting insertfurther has a flat cutting edge extending substantially perpendicularlyto the main cutting edge.

According to the invention, a machined bottom face of the workpiece canbe wiped by the flat cutting edge, and the milling tool made by thecutting insert being fitted to the holder can be realized as an endmill. Accordingly it is possible to perform groove-cutting work orshoulder-cutting work on the workpiece.

Furthermore, in the invention, it is preferable that the sectional shapeof the locus of rotation of the main cutting edge cut by an imaginaryplane containing the axis of rotation is an approximate circular arcshape formed by the holder axial direction middle part swelling furtheroutward in the holder radial direction than the holder axial directionend parts, and the holder axial direction center part projects farthestoutward in the holder radial direction.

According to the invention, most of a small cutting amount part of aworkpiece formed by the holder axial direction tip end part of the maincutting edge in a previous cutting operation can be cut by the holderaxial direction middle part of the main cutting edge, and the machinedsurface roughness of the machined wall face after cutting can beimproved further. Also, it is possible to prevent the machined surfaceroughness of the machined wall face after cutting from becoming higheven when, of the holder axis direction end parts of the main cuttingedge, either the holder tip end part or the holder base end part ispositioned outward of the other in the holder radial direction.

Furthermore, in the invention, it is preferable that the sectional shapeof the locus of rotation of the main cutting edge in an imaginary planecontaining the axis of rotation is an approximate circular arc shapeformed by the holder axial direction center part swelling furtheroutward in the holder radial direction than the holder axial directionend parts, and when D (mm) is a holder machining diameter, L (mm) is alength of the straight line joining together the ends of the maincutting edge, θ (°) is an axial rake of the main cutting edge, andX=(D−(D²−L² sin² θ)^(1/2))/2 is a maximum width (mm) between thestraight line joining together the ends of the main cutting edge and animaginary cylindrical surface the same diameter as the machiningdiameter of the holder outward of this straight line in the holderradial direction, then the amount of swell A (mm) of the center part ofthe main cutting edge with respect to the straight line joining the endsof the main cutting edge satisfies the following relationshipX+0.02≦A≦X+0.06.

According to the invention, even in the case of a milling tool of whichthe holder machining diameter, the size of the cutting insert (andparticularly the main cutting edge), and the axial rake of the maincutting edge as of when the cutting insert is mounted to the holder allindividually differ, the shape of the main cutting edge is optimized,and when multi-cut machining is carried out in shoulder-cutting work orgroove-cutting work, the steps arising at the joins between passes as aresult of the machining of multiple passes can be made small, and it ispossible to obtain a high-quality machined surface with a high machinedsurface roughness.

The invention provides a milling tool comprising a cutting insertdescribed above and a substantially cylindrical milling tool holder to acircumferential periphery of which the cutting insert can be removablyfitted.

According to the invention, when the cutting insert is fitted to themilling tool holder, a holder axial direction middle part of the maincutting edge projects further outward in the holder radial directionthan the holder axial direction end parts. Consequently, even if whenthe cutting insert is fitted to the milling tool holder a differencearises a difference in the holder radial direction between distancesfrom the axis of the holder to the holder axial direction end parts ofthe main cutting edge between the holder radial direction positions ofthe holder axial direction end parts of the main cutting edge, whenmulti-cut machining is carried out in shoulder-cutting work orgroove-cutting work, the steps forming at the joins between passes as aresult of the machining of multiple passes can be made small. And themilling tool can be used in the machining of materials where a machinedwall face with a high machined surface roughness is required.

Furthermore, in the invention, it is preferable that the cutting insertis fitted to the holder so that the axial rake of the main cutting edgeis positive.

According to the invention, the cutting resistance applied to the maincutting edge by the workpiece can be made small, and cutting insert canbe used in the machining of materials where a machined wall face with ahigh machined surface roughness is required.

Furthermore in the invention it is preferable that a plurality of thecutting inserts are fitted to the holder.

According to the invention, the cutting resistance encountered by eachcutting insert can be reduced, and the possible depth of cut in theholder radial direction set for the milling tool can be increased. Andas a result of the load applied to each cutting insert when the depth ofcut has been made large being lowered, vibration and bending of theholder can be reduced and the machined wall face after cutting can bemade smoother.

The invention also provides a milling tool of substantially cylindricalshape for cutting a workpiece, comprising a flank formed on an outercircumferential face of the milling tool, a rake face cut in from theflank, and a main cutting edge formed at an intersection ridge line partat the intersection of the rake face and the flank, and when the millingtool is rotated about its axis the tool axial direction middle part ofthe locus of rotation of the main cutting edge projects further outwardin the tool radial direction than the tool axial direction end parts.

According to the invention, it is possible that by touching theworkpiece as it rotates about its axis, the milling tool dicretely cutsthe work piece to perform groove-cutting work and shoulder-cutting work.By so-called multi-cut machining, in which the workpiece is cut atmultiple different levels in the holder axial direction from a surfaceof the workpiece, it is possible to achieve an increased depth ofmachining from a surface to a machined bottom of the workpiece.

In the invention, the tool axial direction middle part of the locus ofrotation of the main cutting edge projects further outward in the toolradial direction than the tool axial direction end parts of the locus ofrotation of the main cutting edge. Accordingly it is possible to obtainthe same effects as those of the milling tool described earlier. Thatis, even if due to variation in the dimensional accuracy of the maincutting edge or bending of the holder a difference arises betweendistances from the axis of the tool to the axial direction end parts ofthe main cutting edge in the tool radial direction, a drop in machinedsurface roughness can be prevented. Accordingly it is possible to makesmall the steps in the machined wall face forming at the joins betweencuts.

The invention also provides a method for cutting a wall face of aworkpiece using a cutting insert according to the invention describedabove, the method comprising a mounting step of making a milling tool byfitting the cutting insert to the holder, an approach step of bringingthe milling tool close to a workpiece, a cutting step of abutting themain cutting edge with a wall face of the workpiece while rotating themilling tool and thereby rotationally cutting the wall face of theworkpiece, and a withdrawal step of moving the milling tool away fromthe workpiece.

According to the invention, in a mounting step a cutting insert isfitted to a milling tool holder to make a milling tool. Then, in anapproach step the milling tool is brought close to a workpiece, in acutting step the workpiece is cut with the milling tool, and in awithdrawal step the milling tool is moved away from the workpiece. Byrotationally cutting a workpiece using a milling tool according to theinvention like this, it is possible to obtain the effects explainedabove. That is, it is possible to improve the machined surface roughnessof the machined wall face after cutting.

Furthermore, in the invention it is preferable that in the cutting step,the stage of rotationally cutting the wall face of the workpiece isperformed a number of times, at different levels in the holder axialdirection.

According to the invention, the steps arising at the joins between cutscan be made small, and the machined surface roughness of the machinedwall face after cutting can be improved. And the milling tool can beused in the machining of materials where a machined wall face with ahigh machined surface roughness is required.

In the invention it is preferable that the method further comprises astep of, after the cutting step of rotationally cutting a wall face ofthe workpiece, rotationally finish-cutting the wall face of theworkpiece with a milling tool for finishing, having a main cutting edgeformed integrally with a circumferential periphery thereof.

According to the invention, rough cutting is carried out with a millingtool having a cutting insert, and a finishing cut is carried out with amilling tool for finishing. As described above, by using a cuttinginsert according to the invention it is possible to improve the machinedsurface roughness of a machined wall face after cutting in arough-cutting stage, and it is possible to make the depth of cut in afinishing cut stage small. Accordingly the cutting time can beshortened. And even if the main cutting edge formed on the cuttinginsert breaks as a result of the depth of cut being made large at thetime of rough cutting, it is possible to restore the sharpness of themilling tool by replacing the cutting insert.

BRIEF DESCRIPTION OF THE DRAWINGS

Other and further objects, features, and advantages of the inventionwill be more explicit from the following detailed description taken withreference to the drawings wherein:

FIG. 1 is a view of an end mill according to a first embodiment of theinvention being rotated about its axis of rotation, showing exaggeratedthe locus of rotation of a main cutting edge of the end mill in asection parallel to the axis of rotation;

FIG. 2 is an overall perspective view of the end mill according to thefirst embodiment of the invention;

FIG. 3 is a side view of a detail of FIG. 2;

FIG. 4 is a perspective view showing partly exaggerated a cutting insertaccording to the first embodiment of the invention;

FIG. 5 is a plan view showing the cutting insert exaggerated;

FIG. 6 is a side view of the cutting insert;

FIG. 7 is a perspective view illustrating the locus of rotation of amain cutting edge;

FIG. 8 is a view in the direction of the arrow B in FIG. 7;

FIG. 9 is a view in the direction of the arrow C in FIG. 7;

FIG. 10 is a flow chart showing a procedure for cutting a workpieceusing the end mill;

FIG. 11A is a perspective view showing groove-cutting of a workpiecewith the end mill, and FIG. 11B is a perspective view showingshoulder-cutting of a workpiece with the end mill;

FIGS. 12A through 12C are views illustrating a procedure forgroove-cutting a workpiece with the end mill by multi-cut machining,FIG. 12A showing an n-th cut, FIG. 12B showing an n+1-th cut, and FIG.12C showing an n+2-th cut;

FIG. 13 is a front view showing exaggerated a tip end part of the endmill;

FIGS. 14A through 14C are sectional views showing changes in a machinedwall face of a workpiece 11 in multi-cut machining, FIG. 14A showing amachined wall face in an n-th cut, FIG. 14B showing the machined wallface in an n+1-th cut, and FIG. 14C showing the machined wall face in ann+2-th cut;

FIG. 15A is a view showing an end mill according to the invention with adifference in the holder radial direction between distances from theaxis of the holder to the holder axial direction end parts of the maincutting edge, and FIG. 15B is a view showing an end mill of acomparative example with a difference in the holder radial directionbetween distances from the axis of the holder to the holder axialdirection end parts of the main cutting edge;

FIGS. 16A through 16C are sectional views showing changes in a machinedwall face of a workpiece when there is a difference in the holder radialdirection R between distances from the axis of the holder to the holderaxial direction end parts of the main cutting edge of an end millaccording to the invention, FIG. 16A showing the machined wall face inan n-th cut, FIG. 16B showing the machined wall face in an n+1-th cut,and FIG. 16C showing the machined wall face in an n+2-th cut;

FIGS. 17A through 17C are sectional views showing changes in a machinedwall face of a workpiece when there is a difference in the holder radialdirection R between distances from the axis of the holder to the holderaxial direction end parts of the main cutting edge or the end mill ofthe comparative example, FIG. 17A showing the machined wall face in ann-th cut, FIG. 17B showing the machined wall face in an n+1-th cut, andFIG. 17C showing the machined wall face in an n+2-th cut;

FIG. 18A is a sectional view showing exaggerated the machined wall faceof a workpiece multi-cut machined in shoulder-cutting with an end millaccording to the invention, and FIG. 18B is a sectional view showingexaggerated the machined wall face of a workpiece multi-cut machined inshoulder-cutting with an end mill of the comparative example;

FIG. 19 is a flow chart showing a cutting procedure for when a finishingcut is carried out on a workpiece;

FIG. 20 is a view of a cutting insert according to a second embodimentof the invention being rotated about the axis of rotation of a holder,showing exaggerated the locus of rotation of a main cutting edge in asection parallel to its axis of rotation;

FIG. 21 is a view of a cutting insert according to a third embodiment ofthe invention being rotated about the axis of rotation of a holder,showing exaggerated the locus of rotation of a main cutting edge in asection parallel to its axis of rotation; and

FIG. 22 is a side view showing a milling tool of a fourth embodiment ofthe invention.

DETAILED DESCRIPTION

Now referring to the drawings, preferred embodiments of the inventionare described below.

Preferred embodiments of the invention will now be described withreference to the accompanying drawings.

FIG. 1 is a view of an end mill 10 according to a first embodiment ofthe invention being rotated about its axis of rotation 9, showingexaggerated the locus of rotation of a main cutting edge 6 of the endmill 10 in a section parallel to the axis of rotation 9. FIG. 2 is anoverall perspective view of an end mill 10 according to the firstembodiment of the invention. FIG. 3 is a side view of a detail of FIG.2.

An end mill 10 according to the first embodiment of the invention is asubstantially cylindrical milling tool that has a main cutting edge 6 atits circumferential periphery and rotates about an axis of rotation 9.While the end mill 10 is rotated about the axis of rotation 9 by amilling machine or the like, the main cutting edge 6 makes contact witha workpiece and discretely cuts the workpiece. Accordingly it ispossible to cut the workpiece to a predetermined shape.

The end mill 10 of this embodiment is made up of a cutting insert(hereinafter abbreviated to insert) 2 on which are formed a long-sidecutting edge 51 constituting the main cutting edge 6 and a short-sidecutting edge 52 constituting a flat cutting edge 7, and an end millholder (hereinafter abbreviated to holder) 1 to which the insert 2 isremovably fitted. In this embodiment the holder 1 is constructed so thata plurality of for example three inserts 2 can be fitted to it.

The holder 1 is formed in an approximately cylindrical shape. A chuckingpart to be held by arbors to a miller such as a milling center is formedat a base end part 53 of the holder 1. A fitting part to which theinserts 2 are fitted is formed at a tip end part 54 of the holder 1.

As shown in FIG. 2 and FIG. 3, when each insert 2 is fitted to theholder 1, the long-side cutting edge 51 of the insert 2 projects outwardin the radial direction of the holder from the circumferential face 55of the holder 1 to form the main cutting edge 6. And the short-sidecutting edge 52 of the insert 2 projects outward in the holder axialdirection from a tip end face 56 of the holder 1 to form the flatcutting edge 7.

As shown in FIG. 1, the main cutting edge 6 extends in the holder axialdirection X of the holder 1 and projects from the circumferential face55 of the holder 1 in the outward holder radial direction R2. When theend mill 10 rotates about the axis of rotation 9, the main cutting edge6 describes a predetermined locus of rotation 22. Of this locus ofrotation of the main cutting edge, a holder axial direction middle part60 projects further in the outward holder radial direction R2 thanholder axial direction end parts 61, 62.

As shown in FIG. 2 and FIG. 3, a channel is formed in the tip end partof the holder 1 coming in through the circumferential face and the tipend part end face of the holder 1. This channel is a space made up of aninsert-receiving space and a chip pocket 14. The insert-receiving spaceis a space that receives almost all of the insert 2. The chip pocket 14is a space for temporarily receiving chips produced by the cutting edges51, 52 of the insert 2.

The fitting part of the holder 1 has a seat face 98 with which a bottomface of the insert 2 abuts and a wall face rising from the seat face 98.The seat face 98 angles in from the tip end face 56 of the holder 1 andextends in a back holder axial direction X2 toward the base end part 53,and also angles in from the circumferential face 55 of the holder 1 andextends in the inward holder radial direction R1. As shown in FIG. 3,with progress toward the base end part 53 the seat face 98 slopes at apredetermined slope angle α1 in the opposite direction to the rotationdirection of the holder 1 with respect to a plane 99 parallel to theaxis of rotation 9.

The insert 2 has its bottom face abutting with the seat face 98 of theholder 1 and has at least one of its side faces abutting with a wallface of the holder 1. In this state, the insert 2 is fixed to the holder1 by a clamping member such as a screw member.

FIG. 4 is a perspective view showing partly exaggerated the cuttinginsert 2 according to the first embodiment of the invention. FIG. 5 is aplan view showing the cutting insert 2 exaggerated. FIG. 6 is a sideview of the cutting insert 2.

The cutting insert 2 is substantially plate-shaped. The cutting insert 2has corner R cutting edges 5 formed at two diagonally opposite corners70, 72 among the four corners 70 to 73 of an intersection ridge 24formed by a side face 3 and an upper face 4 of the cutting insert bodythat is approximately parallelogram-shaped in plan view, and having along side cutting edge 51 and a short side cutting edge 52 on oppositesides of each of these corner R cutting edges 5.

On the upper face of the cutting insert 2, to smoothly remove chipsproduced by end milling such as shoulder and grooving work, a rake face8 with a fixed rake angle is formed along the long side cutting edge 51.In plan view, the insert 2 has a middle part 82 of its long-side cuttingedge 51 projecting further in the width direction C than end parts 80,81 of the long-side cutting edge 51. And in side view, the long-sidecutting edge 51 of the insert 2 approaches the bottom face 59 of theinsert 2 with progress along it in the length direction B away from acorner R cutting edge 5.

Explaining this in more detail, the insert 2 is formed in an approximateplate shape. When the insert 2 is projected onto a projection planeperpendicular to its thickness direction A, on that projection plane,the insert 2 is approximately parallelogram-shaped, and of its two pairsof sides one pair of sides is longer than the other.

A through hole 90 is formed in the insert 2, passing through it in itsthickness direction A. The through hole 90 is disposed in a centralposition in the length direction B and the width direction C. Thethrough hole 90 is a hole for fixing the insert 2 to the holder 1.

In this embodiment, with the axis 91 of the through hole 90 as areference axis, the insert 2 is 180°-rotationally symmetrical in shapeabout this reference axis 91; in other words, it is twice rotationallysymmetrical.

Of an upper side of the insert 2 on a first thickness direction A1 sidethereof, a long-side cutting edge 51 is formed at each of a pair ofmutually opposite long-side edges. And of the same upper face, ashort-side cutting edge 52 is formed at each of a pair of mutuallyopposite short-side edges. The cutting edges 51, 52 are formed atintersection ridge lines where the upper face 4 and side faces 3 of theinsert 2 intersect. The long-side cutting edges 51 are formed oversubstantially the whole of the length direction B of the insert 2.

Each of the cutting edges 52 is formed over a part of the widthdirection C of the insert 2. The radius of curvature of each of thecorner R cutting edges 5 is set to 0.4 mm or more and 4.0 mm or less. Abottom face 59 perpendicular to the reference axis 91 is formed on abottom side of the insert 2 on a second thickness direction A2 sidethereof. This bottom face 59 abuts with the seat face 98 of the holder 1when the insert 2 is fitted to the holder 1.

As shown in FIG. 5, when the insert 2 is seen in plan view, the lengthdirection middle part 82 of each long-side cutting edge 51 projectsfurther out in the width direction C than the length direction end parts80, 81. That is, in plan view, of each of the long-side cutting edges51, the length direction middle part 82 projects outward in the widthdirection C of an imaginary straight line 18 joining the lengthdirection end parts 80, 81. And the length direction middle part 82 ofeach of the long-side cutting edges 51 projects outward in the widthdirection C of a predetermined imaginary curved line 17.

This imaginary curved line 17 is the curved line along which thelong-side cutting edge 51 extends when the insert 2 is seen in plan viewin the case where, with the insert 2 fitted to the holder 1, the locusof rotation of the main cutting edge is a cylinder. In other words, itis the curved line along which the long-side cutting edge 51 extendswhen the insert 2 is seen in plan view in the case where, when the locusof rotation 22 of the main cutting edge 6 is cut by an imaginary planecontaining the axis of rotation 9, that cross-section of the locus ofrotation of the main cutting edge 6 extends in a straight line.

And as shown in FIG. 6, each of the long-side cutting edges 51 slopestoward the bottom face 59 with progress along it in the length directionB away from the respective corner R cutting edge 5. In other words, ofthe insert 2, the thickness direction A dimension of each of the sidefaces 3 that are roughly perpendicular to the width direction Cdecreases with progress away from the respective corner R cutting edge 5in the length direction B.

Therefore, a plane 92 perpendicular to the reference axis 91 makes apredetermined twist angle α2 with each of the long-side cutting edges51. Consequently, when the insert 2 has been fitted to the holder 1, themain cutting edge 6 slopes toward the bottom face 59 of the insert 2with progress from the holder tip end part 54 toward the holder base endpart 53. In this way, the long-side cutting edges 51 are formed in atwist shape having a twist angle α2. (In fact, it is sufficient if thelong-side cutting edges 51 are roughly sloping in side view.Accordingly, besides extending in a straight line, each of the long-sidecutting edges 51 may alternatively extend in a gentle curve shape or maybe made up of a combination of a plurality of straight lines or curves.)

As mentioned above, the seat face 98 of the holder 1 slopes at apredetermined slope angle α1. And the long-side cutting edge 51 of theinsert 2 slopes at a predetermined twist angle α2. Therefore, as shownin FIG. 3, when the insert 2 is fitted in the holder 1, the axial rake θof the main cutting edge 6 is positive. That is, with progress towardthe holder base end part 53, with respect to a straight line parallelwith the axis of rotation 9, the main cutting edge 6 moves away in theopposite direction to the rotation direction of the holder 1. As aresult of the axial rake θ being positive like this, the cuttingresistance encountered by the main cutting edge 6 can be lowered, andthe sharpness of the main cutting edge 6 in end milling work can beimproved.

With respect to the positive axial rake angle θ, when, in a side view ofthe cutting insert, the main cutting edge ridge line consists of asingle straight line, the angle made by that straight line and the axisof rotation of the holder can be taken as θ; however, when, in a sideview of the cutting insert, the main cutting edge ridge line consists ofa combination of a plurality of straight lines or curves, the ends ofthe main cutting edge 6 (the boundaries with the corners) are connectedby a straight line and the angle made by that straight line and the axisof rotation 9 of the holder is taken as θ.

The axial rake θ is the value (α1+α2) obtained by adding the slope angleα1 to which the seat face 98 of the holder 1 is set to the twist angleα2 to which the long-side cutting edge 51 of the insert 2 is set. Inthis embodiment, because the slope angle α1 and the twist angle α2 areboth positive, the axial rake θ can be made large, and the cuttingresistance can be lowered further. And as long as the axial rake θ ispositive, one or the other of the slope angle α1 and the twist angle α2may alternatively be made zero.

In an end mill 10 according to the invention, as shown in FIG. 1, themain cutting edge 6 is made to curve further than the milling tool ofJapanese Patent No. 3085686, and the main cutting edge 6 is formed in asubstantially cylindrical shape swelling outward in the holder radialdirection with progress from either end 61, 62 of the main cutting edge6 toward its center 63. In other words, a sectional shape of the locusof rotation of the main cutting edge 6 cut by an imaginary planeincluding the axis of rotation 9 and extending along the axis ofrotation 9 is not a straight line but rather an approximate circulararc.

As a result, when the cutting insert 2 is rotated while fitted in theholder 1, even if a difference arises in the turning radii of the holderaxial direction end parts 61, 62 of the main cutting edge 6, the stepsproduced between the passes by multi-cut machining in shoulder work orgrooving work can be made small, and consequently high-quality productmachining with high machined surface roughness becomes possible.

Explaining this in more detail, as shown in FIG. 1, the locus ofrotation 22 of the main cutting edge 6 is an approximate cylinder shapein which the holder axial direction middle part 60 swells convexlyfurther outward in the holder radial direction than the holder axialdirection end parts 61, 62. In this embodiment, the sectional shape ofthe locus of rotation 22 of the main cutting edge 6 when cut by theabove-mentioned imaginary plane is an approximate circular arc shapethat swells outward with progress from the holder axial direction endparts 61, 62 of the locus of rotation of the main cutting edge towardthe center part 63 of the locus of rotation of the main cutting edge.That is, the center part 63 of the locus of rotation 22 of the maincutting edge 6 projects farthest in the outward holder radial directionR2.

Here, the middle part 60 of the locus of rotation of the main cuttingedge 6 is the section excluding the end parts 61, 62 of the locus ofrotation of the main cutting edge 6. And the center part 63 of the locusof rotation of the main cutting edge 6 is a portion of the middle part60 and is the section in the holder axial direction center of the locusof rotation of the main cutting edge 6.

In the locus of rotation 22 of the main cutting edge 6, the amount ofswell w in the radial direction constituting the outward holder radialdirection R2 dimension from the end parts 61, 62 to the center part 63is set to 0.02 mm or more and 0.06 mm or less. And when a minimum valueof the allowed radial direction swell amount w is written w1, a maximumvalue of the allowed radial direction swell amount w is written w2, andthe holder axial direction dimension between the end parts 61, 62 iswritten m, then the radius of curvature s of the locus of rotation ofthe main cutting edge is set to at least {(w1)²+(m/2)²}/(2.w1) and notgreater than {(w2)²+(m/2)²}/(2.w2).

FIG. 7 is a perspective view for illustrating the locus of rotation 22of the main cutting edge 6. FIG. 8 is a view in the arrow B direction inFIG. 7. FIG. 9 is a view in the arrow C direction in FIG. 7. Using FIG.7 to FIG. 9, the locus of rotation 22 of the main cutting edge 6 of thisembodiment will now be explained.

The end parts 61, 62 of the main cutting edge 6 are disposed apart fromeach other in the holder axial direction X. And in an ideal state, theend parts 61, 62 of the main cutting edge 6 are both the same radialdirection distance (D/2) from the axis of rotation 9. And because theaxial rake θ of the main cutting edge 6 is set to be positive, as shownin FIG. 9, the holder base part end 61 of the main cutting edge 6 ispositioned downstream of the holder tip part end 62 in the end millrotation direction R.

The cylinder formed by a circle 86 made when the end parts 61, 62 of themain cutting edge 6 complete a rotation about the axis of rotation 9extending in the holder axial direction X will be called the imaginarycylinder 15. In this case, the imaginary straight line is joiningtogether the holder base part end 61 and the holder tip part end 62proceeds from the holder base part end 61 to the holder tip part end 62on the inner side of the outer surface of the imaginary cylinder 15. Andwhen an imaginary plane 16 containing the upper face 4 of the insert 2is set, this imaginary plane 16 includes the imaginary straight line 18.

A curved line extending in the imaginary plane 16 from the holder basepart end 61 of the main cutting edge 6 to the holder tip part end 62will be called the imaginary curved line 17. In the invention, themiddle part 60 of the main cutting edge 6 extends in the imaginary plane16 and curves further outward in the holder radial direction than theimaginary curved line 17 in an approximate circular arc shape.

Here, a view of the insert 2 seen in a direction perpendicular to theimaginary plane 16 shown in FIG. 7 is the plan view of the insert 2 inFIG. 5. That is, the imaginary straight line 18 and the imaginary curvedline 17 shown in FIG. 7 and FIG. 9 are respectively the imaginarystraight line 18 and the imaginary curved line 17 shown in FIG. 5. Andthe amount of radial direction swell w mentioned above is the holderradial direction distance from the imaginary curved line 17 to thecenter part 63 of the main cutting edge 6.

With respect to the end mill 10 according to the invention, when theholder machining diameter is written D (mm), the length of the imaginarystraight line 18 joining together the ends 61, 62 of the main cuttingedge 6 is written L (mm), the axial rake angle of the main cutting edgeridge 6 is written θ (°), and a maximum width between the imaginarystraight line 18 and the imaginary cylindrical plane 16 is written X, anamount of swell A (mm) which indicates a swell of the center 64 of themain cutting edge in the holder radial direction with respect to theimaginary straight line 18 satisfies the following relationshipX+0.02≦A≦X+0.06.

Herein, the maximum width X between the imaginary straight line 18 andthe imaginary cylindrical plane 16 satisfies the following relationship(D−(D²−L² sin² θ)^(1/2))/2.

In FIG. 9, providing the maximum width of the imaginary curved line 17in the holder radial direction with respect to the imaginary straightline 18 is written X, then the following relationship is established:D ²=(D−2X)²+(L sin θ)².That is,X=(D−(D ² −L ² sin² θ)^(1/2))/².

The construction that the main cutting edge 6 extends along theimaginary curved line 17 is a construction that the long-side cuttingedge 51 of the cutting insert 2 is disposed on an imaginary cylinder,i.e. the construction shown in Japanese Patent No. 3085686. In theinvention, the main cutting edge 6 extends to curve further out in theholder radial direction than the imaginary curved line 17. Therefore, inthe invention, compared to the main cutting edge shown in JapanesePatent No. 3085686, the center part of the main cutting edge 6 projectsfurther outward in the holder radial direction.

FIG. 10 is a flow chart showing a procedure for cutting a workpieceusing the end mill 10. The end mill 10 is fitted to a milling machine orthe like to cut a workpiece.

A milling machine includes translation driving means for drivingtranslation of a clamped workpiece and a chucked end mill 10 relative toeach other, and rotation driving means for rotating the chucked end mill10 about its axis of rotation 9. The end mill 10 makes contact with theworkpiece while rotating about the axis of rotation 9, and discretelycuts the workpiece with the cutting edges 51, 52 formed on the inserts2. Accordingly the work piece can be machined to a predetermined shape.

When the cutting edges 51, 52 of an insert 2 have worn or broken, aninsert 2 having new cutting edges 51, 52 is fitted to the holder 1 inplace of the insert 2 of which the cutting edges 51, 52 have worn orbroken, and the cutting capability of the end mill 10 is therebyrestored.

In a step a0, an operator prepares for machining operation by clamping aworkpiece to the milling machine and for example inputting a cuttingprogram to the milling machine. When the operator finishes thispreparation, he proceeds to step a1 and starts machining operation. Instep a1 the operator carries out a mounting step of fitting a cuttinginsert 2 to a holder 1 to make an end mill 10 and fitting the end mill10 to the milling machine, and then proceeds to step a2.

In step a2, with the milling machine, rotation of the end mill 10 isstarted. In step a3, with the milling machine, an approach step ofbringing the end mill 10 close to a cutting position where the workpieceis to be cut is carried out, and then proceeds to step a4.

In step a4, with the milling machine, the rotating end mill 10 isbrought into contact with the workpiece, and then in this contactingstate the end mill 10 and the workpiece are moved translationallyrelative to each other in a direction orthogonal to the axis ofrotation. In this way, a cutting step of rotationally cutting theworkpiece with the end mill 10 is carried out, and then proceeds to stepa5.

In step a5, when the end mill 10 has moved to a predeterminedtranslation end position, with the milling machine, a withdrawal step ofmoving the end mill 10 away from the workpiece is carried out. In stepa6, it is determined by an operator or the like whether or not thepredetermined cutting operation has been completed. When it isdetermined that the predetermined cutting operation has been completed,the procedure moves on to step a7. And when it is determined that thecutting operation has not been completed, to carry out the next cuttingoperation the procedure returns to step a3 and the end mill 10 is movedto a cutting start position for carrying out the next cutting operation.

In step a7, rotation of the end mill 10 is stopped. In step a8, cuttingof the workpiece with the end mill 10 is ended.

For example when multi-cut machining is carried out to shoulder-cut orgroove-cut the workpiece, in the cutting completion determination stepa6 it is determined whether or not a number of cuts based on apredetermined depth of cut have been made, and cutting operation isrepeated until the number of cuts reaches a predetermined number.

FIG. 11A shows a cutting state of groove-cutting and FIG. 11B a cuttingstate of shoulder-cutting. FIGS. 12A through 12C are views illustratinga multi-cut machining procedure for groove-cutting a workpiece with anend mill 10.

In the case of groove-cutting, the end mill 10, with its tip end part 54fed to below the upper face 13 of the workpiece by a predetermined axialdirection cut depth, makes contact with the workpiece 11 from a sideface 12 of the workpiece, and moves in a direction orthogonal to theaxis of rotation 9. The end mill 10 has a pre-set axial direction depthof cut that the end mill is capable of cutting in one cutting operation.

For example when a groove of the required depth cannot be cut in onecutting operation, cutting operations are carried out one after anotherin the order shown in FIGS. 12A through 12C, and the cut workpiece 11 iscut further from its upper surface. That is, multiple cutting operationsof rotationally cutting the wall face of the workpiece 11 are carriedout at different levels in the holder axial direction. By multi-cutmachining being carried out like this, a groove of the required depthcan be formed. And similarly in the case of shoulder-cutting, theworkpiece can be cut to the required shape.

FIG. 13 is a view showing exaggerated the tip end part 54 of an end mill10, and FIG. 14 is a sectional view showing changes of the machined wallface of a workpiece 11 in multi-cut machining. Of the main cutting edge6, the holder axial direction center part 63 projects further in theoutward holder radial direction R2 than the holder axial direction endparts 61, 62.

As shown in FIG. 14A, when an n-th cut is carried out, the machined wallface is cut in a shape following a curved surface 150 according to thelocus of rotation of the main cutting edge 6. Then, as shown in FIG.14B, in an n+1-th cut, the cutting is carried out in a state where theend mill 10 is inserted in an axial direction X1 compared to the n-thcut. And a part to be cut 111 of the workpiece having been cut in then-th cut by the holder axial direction tip end part 62 of the maincutting edge, is cut by the holder axial direction middle part 60 of themain cutting edge 6. The holder axial direction base part 61 of the maincutting edge 6 passes through a vicinity of a part to be cut 113 of theworkpiece having been cut in the n-th cut by the holder axial directionmiddle part 60 without cutting. As shown in FIG. 14C, in the n+1-th cut,the cutting is carried out in a state where the end mill 10 is insertedin an axial direction X1 compared to the previous stage. By repeatingthe cutting in such a manner, it is possible to form a deep groove(shoulder) portion in the workpiece 11.

Here, it is desirable for the axial direction depth of cut of the endmill 10 in one cutting operation to be set to below the holder axialdirection dimension m of the main cutting edge 6, and still morepreferably to be set to below half of the axial direction dimension m ofthe main cutting edge 6. This makes it possible to remove, with theholder axial direction middle part 60 of the main cutting edge 6, moreof the part to be cut of the workpiece have been cut in the previouscutting operation by the holder axial direction tip part 61.

FIG. 15A is a view showing a state wherein there is a difference in theholder radial direction R between distances from the axis of the holderto the holder axial direction end parts 61, 62 of the main cutting edge66 of an end mill 10 according to the invention. FIG. 15B is a viewshowing a state wherein there is a difference in the holder radialdirection R between distances from the axis of the holder to the holderaxial direction end parts 261, 262 of the main cutting edge 206 of anend mill 210 of a comparative example. In the end mill 210 of thecomparative example, the main cutting edge 206 extends along theimaginary curved line 17 discussed earlier.

Due to dimensional error of the insert 2, 202, error in mounting of theinsert 2, 202 to the holder 1, 201, and bending of the holder 1, 201, itis difficult to make the difference in the holder radial direction Rbetween distances from the axis of the holder to the holder axialdirection end parts 61, 62 or 261, 262 of the main cutting edge 6 or 206zero.

For example, on the insert itself, In a state where the insert isattached to the holder 1, 201, variation arises in a size difference inthe holder radial direction between the holder axial direction tip part62 and the holder axial direction base part 61 of the main cutting edge6, 206 over a range of ±50 μm. And as bending of the end mill 10, 210,bending of at most 0.3 mm in the holder radial direction R may arise inthe holder 1, 201 during cutting.

In the invention, even if a difference R in the holder radial directionhas arisen at the holder axial direction end parts 61, 62 of the maincutting edge 6, a part of the holder axial direction middle part 60 ofthe main cutting edge 6 projects further out in the holder radialdirection than the holder axial direction end parts 61, 62. That is, theradial direction dimension L3 of a part of the holder axial directioncenter part 63 is greater than the radial direction dimensions L1, L2 ofthe holder axial direction end parts 61, 62.

With respect to this, in the comparative example, as a result of adifference arising in the holder radial direction R between distancesfrom the axis of the holder to the holder axial direction end parts 261,262 of the main cutting edge 206, one or the other of the holder axialdirection end parts 261, 262 of the main cutting edge 206 projectsfurther in the outward holder radial direction R2 than the holder axialdirection center part 263. That is, one or the other of the radialdirection dimensions L1, L2 of the holder axial direction end parts 261,262 is larger than the radial direction dimension L3 of the holder axialdirection center part 263.

FIGS. 16A through 16C are sectional views showing changes of a machinedwall face of a workpiece 11 when there is a difference in the holderradial direction R between distances from the axis of the holder to theholder axial direction end parts 61, 62 of the main cutting edge 6 in anend mill 10 according to the invention. FIGS. 17A through 17C aresectional views showing changes of a machined wall face of a workpiecewhen there is a difference in the holder radial direction R betweendistances from the axis of the holder to the holder axial direction endparts 261, 262 of the main cutting edge 6 in an end mill 210 of thecomparative example.

As shown in FIGS. 16A through 16C and FIGS. 17A through 17C, when adifference in the holder radial direction R between distances from theaxis of the holder to the holder axial direction end parts 61, 62 or261, 262 of the main cutting edge 6 or 206 has arisen, compared to thecase where the difference in the holder radial direction betweendistances from the axis of the holder to the holder axial direction endparts 61, 62 or 261, 262 of the main cutting edge 6, 206 is zero, at oneof the holder axial direction end parts 61, 62 or 261, 262 a cuttingamount increases, and at the other an cutting amount decreases.

In the invention, compared to the comparative example, the holder axialdirection middle part 60 of the main cutting edge 6 removes more of theworkpiece. Accordingly, it is possible to reduce steps 160 producedbetween the passes by the multi-cut machining that arise due to thedifference in the cutting amount between the holder axial direction endparts of the main cutting edge 6. Consequently, the affect on themachined surface roughness of the cut machined wall face of theabove-mentioned difference in the holder radial direction R positions ofthe holder axial direction end parts 61, 62 of the main cutting edge 6can be made small.

In contrast to this, in the comparative example, because the holderradial direction dimension L3 of the holder axial direction middle part260 of the main cutting edge 206 is smaller than the holder radialdirection dimension L1 or L2 of one or the other of the holder axialdirection end parts 261, 262, the workpiece cannot be fully removed bythe holder axial direction middle part 260. In other words, of the maincutting edge 206, the holder axial direction end part 261 having alarger radial direction dimension than the holder axial direction middlepart 260 cuts the workpiece excessively. Consequently, in the machinedwall face of the workpiece cut with the end mill 210 of the comparativeexample, large steps form at the joins 215 in multi-cut machining.

As explained above, as shown in FIGS. 16A through 16C and FIGS. 17Athrough 17C, compared to the comparative example, with the invention itis possible to make the machined surface roughness of the machined wallface of the workpiece 11 after cutting higher.

FIG. 18A is a schematic sectional view of a machined wall face of aworkpiece multi-cut machined in shoulder work with an end mill accordingto the invention, and FIG. 18B is a schematic sectional view of amachined wall face of a workpiece multi-cut machined in shoulder workwith an end mill of the comparative example. In the end mill 210 of thecomparative example shown in FIG. 18B, in an ideal situation where nodifference arises between the turning radii of the ends 261, 262 of themain cutting edge 206, there is no formation of a step on the machinedwall face.

However, on close examination it often happens that due to variation ofthe fitting accuracy of the cutting insert 2, 202 and the holder 1 orvariation of dimensional accuracy or the like the cutting insert isfitted with the main cutting edge slanting so that one of its endsprojects further outward than the other, and when multi-cut machining iscarried out with an end mill in this kind of state, a step matching thelocus of rotation of the cutting edge on each pass forms in the machinedwall face of the workpiece in the end mill 210 of the comparativeexample.

Specifically, supposing that a cutting insert according to the inventionand a cutting insert of the comparative example are fitted slanting atthe same angle, then the state of the resulting machined face at the endof the cutting edge will be the shape shown in FIGS. 18A and 18Brespectively. That is, of the loci of rotation 22, 23 of the maincutting edge shown with dashed lines in FIGS. 18A and 18B, it can beseen that in the case of the locus of rotation 22 of the main cuttingedge according to the invention, more than in the case of the locus ofrotation 23 of the main cutting edge of the comparative example, themain cutting edge inserts more into the workpiece wall face as a resultof the central part of the main cutting edge being swollen outward, andwhen multi-cut machining is carried out repeatedly the step h1 in themachined wall face arising between the passes is smaller in the case ofthe main cutting edge according to the invention than the step h2 in thecase of the main cutting edge of the comparative example, and the wallface machined with the end mill of the invention has smaller steps andhigher roughness of the machined face at the machining end part also, asis clear from FIGS. 18A and 18B. Here, the step h₁, h₂ in the machinedwall face means the step between the holder axial direction tip end ofthe cut trace made by the present cutting operation and the holder axialdirection base end of the cut trace made by the cutting operationfollowing the present cutting operation.

For example as cutting conditions of multi-cut machining, the cuttingspeed was made 120 m/min, the feed per 1 tooth was made 0.1 mm, theradial direction depth of cut was made 10 mm, the axial direction depthof cut was made 5 mm, the length L of the main cutting edge 6 in planview was made 10 mm, the axial rake of the main cutting edge 6 was made20°, a carbon steel for machine structure use S50C specified in JIS G4051 was used as the workpiece, and during cutting no cutting liquid wasused but air blowing was carried out. The machined wall face step h of aworkpiece cut under these cutting conditions is shown in the followingtable.

END MILL TYPE STEP h End mill of the invention 14 μm End mill with maincutting edge on imaginary 33 μm line 17 End mill with main cutting edgeon imaginary 52 μm line 18

As this table shows, it was found in experimental results that the steph in the machined wall face is smaller with the end mill 10 of theinvention than with the end mill 210 of the comparative example.

The amount w of swell in the holder radial direction from the holderaxial direction end parts to a projection part projecting most outwardin the holder radial direction of the holder axial direction middle partwith respect to the locus of rotation of the main cutting edge 6 ispreferably 0.02 mm or more and 0.06 mm or less. Accordingly, even ifthere is a certain amount of variation in the mounting state of thecutting insert 2, machining steps can be kept small and an amplemachined surface roughness increasing effect can be obtained.

Specifically, when the amount of swell w in the holder radial directionR is less than 0.02 mm, variation in the main cutting edge positionarising when the insert 2 is fitted to the holder 1, that is, thedifference in the holder radial direction between distances from theaxis of the holder to the holder axial direction end parts 61, 62 of themain cutting edge 6, cannot be absorbed, and the steps at the joinsbetween successive cuts become large. And when the amount of swell w inthe holder radial direction R exceeds 0.06 mm, due to the affect of thisswell in the holder radial direction R of the main cutting edge 6, themachined wall face becomes too concave, and the steps at the joinsbetween successive cuts become large as a result of that.

With respect to this, with the invention, by the amount of swell w inthe holder radial direction R being made 0.02 mm or more and 0.06 mm orless, without the holder radial direction R swell w becoming too large,a holder radial direction R difference between distances from the axisof the holder to the holder axial direction end parts 61, 62 of the maincutting edge 6 can be absorbed. Accordingly it is possible to make thesteps arising at the joins between successive cuts small and to stablyobtain a high-quality machined wall face.

Also, by forming corner R cutting edges 5 of radius of curvature r being0.4 mm or more and 4.0 mm or less at the corner R parts continuous withthe main cutting edges 6, the step at the end of the main cutting edgecan be made smaller and the machined surface roughness increased.

In the end mill 10 according to the invention, when the amount of swellA (mm) satisfies the following relationship X+0.02≦A≦X+0.06, even in thecase of a milling tool of which the holder machining diameter, the sizeof the cutting tool insert (the main cutting edge), and the axial rakeangle of the main cutting edge ridge line as of when the cutting toolinsert is mounted to the holder all individually differ, the shape ofthe main cutting edge ridge line is optimized, and when multi-cutmachining is carried out in shoulder work or grooving work, the stepsarising between passes as a result of the machining of multiple passescan be made small.

In addition, in the end mill shown in Japanese Patent No. 3085686, asmentioned above, due to variation of the fitting accuracy between theholder and the cutting insert or variation of dimensional accuracy, adifference arises between the turning radii of the ends of the maincutting edge when the cutting insert is mounted to the holder, and whenmulti-cut machining is carried out in shoulder work or grooving work,large steps arise at the joins between passes in the machining ofmultiple passes in the machined wall face.

With respect to this, in the end mill according to the invention, theamount of swell A satisfies the following relationship X+0.02≦A≦X+0.06.Consequently, the sectional shape of the locus of rotation of the maincutting edge 6 is not a straight line but rather is an approximatecircular arc, swelling with progress toward its center 63. As a result,in real machining, where there is a difference between the turning radiiof the ends 61, 62 of the main cutting edge, it is possible to make thesteps arising in the machined wall face small as described above.

By this kind of relationship being satisfied, the sectional shape of thelocus 22 of rotation of the main cutting edge 6 always takes a suitablecircular arc shape, irrespective of the holder diameter D, the length Lof the insert 2, or the amount of the axial rake angle θ, and a stablemachined surface roughness can be obtained.

It is desirable for an insert 2 with a different holder radial directionR swell shape of the locus of rotation 22 of its main cutting edge 6 tobe prepared for each diameter D of the holder and size L of the insert2. However, if the repertory of inserts 2 is made overly large, itsmanagement becomes complicated. So, a swell shape of the main cuttingedge 6 such that unevenness of the machined wall face is made small isdesigned. And in a case where inserts 2 is fitted in holders ofdifferent diameters from holders used frequently, the design is made soas to adjust the axial rake and radial rake of the main cutting edge 6by changing an inclination of the seat face of the holder, and make theunevenness small without changing the shape of the insert 2. Accordinglyit is possible both to manage and design multiple inserts easily and toform machined wall faces having small unevenness on workpieces.

In this embodiment, the holder axial direction center part 63 swellsfurther out in the holder radial direction R than the holder axialdirection end parts 61, 62 to form an approximate circular arc shape.Accordingly, most of the part to be cut of the workpiece in which itscutting amount is small formed by one of the holder axial direction endparts 61, 62 of the main cutting edge 6 in a previous cutting operationcan be cut by the holder axial direction middle part 63 of the maincutting edge 6, and the machined surface roughness of the machined wallface after cutting can be improved further.

And in this embodiment, the holder axial direction center part 63 of themain cutting edge 6 projects farthest in the outward holder radialdirection R2. As a result of this, whether the holder axial directiontip end part 62 of the main cutting edge 6 projects further outward inthe holder radial direction R2 than the holder axial direction base endpart 61 or the holder axial direction base end part 61 projects furtheroutward in the holder radial direction R2 than the holder axialdirection tip end part 62, in either case the machined surface roughnessof the machined wall face can be prevented from becoming high. This isparticularly useful in cases where it is not possible to ascertain whichof the holder tip end part 62 and the holder base end part 61 of themain cutting edge 6 is positioned further outward in the holder radialdirection R2.

In this embodiment, the insert 2 has a bottom face 59 that abuts with aseat face 98 of the holder 1 when the insert 2 is fitted to the holder1, and the long-side cutting edge 51 constituting the main cutting edge6 slopes toward this bottom face 59 with progress from the holder tipend part 54 toward the holder base end part 53. That is, the insert 2 isformed in a twist shape having a twist angle α2. Accordingly, even whenthe degree of slope at which the seat face 98 of the holder 1 slopeswith respect to the axis of rotation 9 is small, a large axial rake canbe provided. In this way it is possible to lower the cutting resistanceencountered by the main cutting edge 6 during cutting. And because inthis embodiment a flat cutting edge extending approximatelyperpendicularly to the main cutting edge 6 is further provided, by theinsert 2 being fitted into the holder 1 an end mill can be realized.

In this embodiment, a plurality of cutting inserts 2 are each fitted tothe holder 1. Accordingly, the cutting resistance encountered by eachcutting insert 2 can be reduced, and the possible depth of cut in theholder radial direction R and the holder axial direction X set for themilling tool 10, and the feed of the end mill 10 can be increased. Andby the load applied to each cutting insert 2 being lowered when thedepth of cut and the feed have been made large, vibration and bending ofthe holder 1 can be reduced and the machined surface roughness of themachined wall face after cutting can be made smoother.

FIG. 19 is a flow chart showing a cutting procedure to be followed whena finishing cut is to be carried out on a workpiece. After a wall faceof a workpiece 11 is cut with an end mill 10 according to the invention,the wall face of the workpiece 11 may be finish-cut with a milling toolfor finishing having a main cutting edge formed integrally with itscircumferential periphery.

In this case, when preparation for cutting work such as preparing theworkpiece has been finished first in a step b0, in a step b1, cuttingwork is started. In step b1, one or multiple cutting operations arecarried out with an end mill 10 incorporating an insert 2, whereby theworkpiece is cut roughly, and when rough cutting is finished, theprocedure moves on to step b2.

In step b2, the workpiece is finish-cut with a milling tool forfinishing having a main cutting edge formed integrally with itscircumferential periphery, for example a solid type end mill. And whenfinish cutting is complete, cutting work is ended in a step b3.

By rough cutting being carried out with an end mill 10 incorporating aninsert 2 like this, even at the rough cutting stage it is possible toimprove the machined surface roughness of the machined wall face aftercutting. Accordingly it is possible to reduce the depth of cut at thefinish cutting stage, and the cutting time of the cutting work can beshortened. And by reducing the amount of cut in the finishing cut stage,it is possible to extend the life of the milling tool for finishing.

When the depth of cut at the time of rough cutting is made large, thereis a possibility of the main cutting edge 6 formed on the insert 2breaking. In the case of that the main cutting edge 6 breaks, thesharpness of the end mill 10 can be restored by a new insert 2 beingfitted to the holder 1.

FIG. 20 is a view showing exaggerated the locus of rotation 422 of amain cutting edge 406 of an insert 402 of a second embodiment of theinvention, in a cross-section parallel to the axis of rotation 409. Inthis second embodiment, the sectional shape of the locus of rotation 422of the insert 402 is a shape different from an approximate circular arc.The rest of the construction is the same as that of the first embodimentand will not be described again here. Parts corresponding to parts inthe first embodiment have been assigned reference numbers obtained byadding 400 to the reference numbers of the first embodiment.

In this second embodiment, the sectional shape of the locus of rotation422 of the main cutting edge 406 slants in the outward holder radialdirection R2 with progress in a straight line from a holder axialdirection tip end part 462 toward a holder axial direction center part463, and slants back in the inward holder radial direction R1 withprogress in a straight line from the holder axial direction center part463 toward a holder axial direction base end part 461.

In this case of the holder axial direction middle part 460 Of the maincutting edge 406 projecting further in the outward holder radialdirection R2 than the holder axial direction base end parts 461, 462also, the same effects as those of the first embodiment can be obtained.

Besides this the sectional shape of the locus of rotation of the maincutting edge may alternatively follow a gentle curve other than acircular arc, or may extend along a line consisting of a combination ofa plurality of straight lines or curves, or may extend in a steppedshape.

FIG. 21 is a view showing exaggerated the locus of rotation of a maincutting edge of an insert 502 of a third embodiment of the invention, ina cross-section parallel to the axis of rotation. In this thirdembodiment, of the locus of rotation of the main cutting edge, a holdertip end side portion 511 of the holder axial direction middle part 560projects farthest in the outward holder radial direction R2. The rest ofthe construction is the same as that of the first embodiment and willnot be described again here. Parts corresponding to parts in the firstembodiment have been assigned reference numbers obtained by adding 500to the reference numbers of the first embodiment.

In the third embodiment, at the location in contact with the workpiece11, the holder axial direction base end part 561 of the main cuttingedge 506 is further inward from the workpiece in the holder radialdirection R than the holder axial direction tip end part 562 of the maincutting edge 506.

In the third embodiment, compared to the rest of the main cutting edge506, a holder tip end side portion 511 of the holder axial directionmiddle part 560 of the main cutting edge 506 projects farthest in theoutward holder radial direction R2. Consequently, even when the holder501 bends during cutting, the projection amount by which the holderaxial direction base end part 561 of the main cutting edge 506 projectsbeyond the holder axial direction tip end part 562 of the main cuttingedge 506 in the outward holder radial direction R2 can be made small.Therefore, the affect of bending of the holder 1 can be made small, andthe machined surface roughness of the machined wall face of theworkpiece 11 can be improved. And it is possible to obtain the sameeffects as those of the first embodiment.

With this third embodiment also, preferably, in the locus of rotation522 of the main cutting edge 506, the amount of swell w in the radialdirection from the holder axial direction tip end parts 561, 562 to theholder tip end side portion 511, which, of the center part 563, projectsfarthest in the outward holder radial direction R2, is 0.02 mm or moreand 0.06 mm or less.

FIG. 22 is a side view showing an end mill 610 of a fourth embodiment ofthe invention. In this fourth embodiment, the main cutting edge 6described in the first embodiment is formed directly on an approximatelycylindrical end mill body. That is, an insert is not used, and the maincutting edge 606 is formed integrally with the body 501.

Specifically, the end mill 610 is formed in ad approximately cylindricalshape. The end mill 610 has a flank 603 formed on its circumferentialface 655, a rake face 608 cut in from the flank 603, and a main cuttingedge 606 formed at the intersection ridge line where the rake face 608and the flank 603 intersect.

When the end mill 610 is rotated about the axis of rotation 609, in thelocus of rotation of the main cutting edge 606, a tool axial directionmiddle part 660 projects further outward in the tool radial directionthan tool axial direction end parts 661, 662. Accordingly it is possibleto obtain the same effects as those of the first embodiment.

Although an embodiment of the invention has been set forth above, theinvention is not limited to the embodiment described here, and variouschanges and modifications can of course be made without deviating fromthe scope of the invention. Although an end mill has been described asan example of a milling tool, the effects described above can also beobtained by applying a main cutting edge according to the invention tomilling tools not having a flat cutting edge, that is, to milling toolsother than end mills.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription and all changes which come within the meaning and the rangeof equivalency of the claims are therefore intended to be embracedtherein.

1. A cutting insert to be fitted to a milling tool holder, comprising:an insert body comprising: an upper face comprising a rake face; a sideface comprising a flank; and a main cutting edge at an intersectionridge line between the rake face and the flank, wherein a locus ofrotation of the main cutting edge is formed when the holder is rotatedaround an axis of the holder with the cutting insert fitted to thecircumferential periphery of the holder, wherein the locus of rotationcomprises a middle part and end parts on both sides of the middle partin a holder axial direction parallel to the axis of the holder, the endparts comprising a tip end part and a base end part which is positionedfurther away from a tip of the holder in the holder axial direction thanthe tip end part, wherein the middle part projects further outward in aholder radial direction perpendicular to the holder axial direction thanthe end parts, and wherein the middle part has a farthest portion whichprojects farthest outward in the holder radial direction and which iscloser to the tip end part in the holder axial direction than the baseend part.
 2. The cutting insert of claim 1, wherein a sectional shape ofthe locus of rotation of the main cutting edge, cut by an imaginaryplane containing the axis of rotation comprises an approximate circulararc shape.
 3. The cutting insert of claim 1, wherein an amount of swellw representing the difference between the largest radius in the middlepart and the smallest radius in the end parts is 0.02 mm or more and0.06 mm or less.
 4. The cutting insert of claim 1, wherein the maincutting edge slopes from the axis of rotation of the main cutting edgeso that the cutting insert has a smaller thickness at the tip end partthan at the base end part.
 5. The cutting insert of claim 1, wherein theinsert body further comprises a corner R cutting edge which is connectedto the main cutting edge, wherein the radius of curvature r of thiscorner R cutting edge is 0.4 mm or more and 4.0 mm or less.
 6. Thecutting insert of claim 1, wherein the insert body further comprises aflat cutting edge substantially perpendicular to the main cutting edge.